Merge branch '372-clean-up-introduction-session-states-when-removing-contact' into 'master'

Clean up Introduction Session States

...for introducer when both introducees have been deleted.

It can't be deleted when only one introducee is removed, because then all messages in the private conversation with the other introducee will disappear, because their corresponding session state can't be found anymore.

Closes #372

See merge request !189